In certain data communications systems, a frame synchronization pattern is periodically inserted into the data stream by the transmitter. A conventional clock and frame recovery scheme at the receiver uses samples of an M-ary phase-shift-keying (MPSK) signal at the output of a Nyquist filter to reconstruct the data clock. The search mechanism used often requires a serial search of the received signal. Upon recovery of the data clock, the received bit or symbol streams are demodulated and the resulting bits or symbols are forwarded to a frame recovery mechanism that searches for a unique word (UW) of length L symbols which follows the synchronization pattern in the preamble of the MPSK signal. The length L of the UW should be long enough so that as compared to the frame length, the probability of the UW appearing in the data stream is minimized.
This scheme requires a reasonably accurate clock timing recovery before frame synchronization (or any form of communication) can take place. [See, for example, T. T. Ha, Digital Satellite Communications, 2.sup.nd edition, McGraw-Hill, New York, 1990, pp. 230-241]. Any false or missed detection of the UW or of the synchronization pattern results in a loss of the data frame that follows the preamble. This in turn may result in large delays in, for example, a Time Division Multiple Access (TDMA) system operating in a less than ideal environment.
Moreover, the above scheme requires a synchronization pattern followed by a fairly long UW sequence to provide for synchronization at the receiver. An additional deficiency of the UW search method used in practice is the sub-optimum nature of the search mechanism. From a detection theory standpoint, the conventional method of recovering the UW of an MPSK signal, which uses a threshold test at the output of the sequence correlator, is a sub-optimum solution to the problem of codeword recovery in the face of additive Gaussian noise. One reason for this simplification is the prohibitively large computational complexity of an optimum codeword search.